The goal of this application is to study how poxviruses escape clearance by the host immune system. Several pathogen recognition receptors are composed of a PYRIN domain, a nucleotide binding domain and leucine rich regions. While the leucine rich region functions as pathogen receptor, the PYRIN domain recruits the adaptor protein ASC through homotypic PYRIN domain interaction. ASC initiates immune responses by activation of downstream effectors, which include caspase-1, NF-KB, and induction of MHC class 1 expression. Impaired regulation of this pathway is also linked to several autoinflammatory disorders. We recently characterized a cellular PYRIN-only protein (POP1) that functions as a dominant negative regulator of the PYRIN domain signal transduction pathway. POP1 interacts with the central PYRIN domain containing adaptor protein ASC, thereby inhibiting signals transmitted by upstream PYRIN domain containing pathogen recognition receptors via ASC to downstream effectors. We provide evidence for functional poxvirus-encoded PYRIN-only proteins. This application seeks to determine, whether poxvirus-encoded PYRIN-only proteins function by suppressing the host immune response following viral infections. We anticipate that viral encoded PYRIN- only proteins are capable to associate with ASC, the central adaptor in the PYRIN domain signal transduction pathway (Aim 1A). We further predict that poxvirus encoded PYRIN-only proteins represent a novel immunoevasive strategy. Viral PYRIN-only proteins might enable poxviruses to suppress effectors of the immune system, including caspase-1, NF-KB, and MHC class 1 expression. We will determine whether inhibition of PYRIN domain signal transduction pathways is selective for a particular effector (Aim 1B). Poxvirus infection results in rapid down regulation of MHC class 1 expression, inhibition of NF-KB, and IL-1beta processing. Since poxviruses could potentially be employed as biological weapons, mechanisms to prevent successful infection are urgently needed. We propose to explore this novel immunoevasive strategy of poxviruses also by gene ablation of the viral PYRIN-only protein of the rabbit myxoma virus, a widely used model for the poxvirus family, and assessment of host cell infection efficiency. (Aim 2). Our goal is to define this novel immunoevasive mechanism of poxviruses, which potentially could be targeted for therapeutic and preventive purposes. By co-evolving with their hosts, poxviruses adopted efficient immunosuppressive strategies, which provide opportunities to better understand and manipulate the immune system for therapy. The here proposed work extends our long-term goal to define pathogen-host interactions and their effect on the immune response. [unreadable] [unreadable]